A conventional active type optical distance measuring sensor is known as a device configured to irradiate light from a light source for projection of light such as an LED (Light Emitting Diode) toward an object, to detect reflected light from the object with a photodetecting element, and thereby to output a signal according to the distance to the object. A PSD (Position Sensitive Detector) is known as an optical distance measuring sensor of the optical triangulation type capable of readily measuring the distance to the object. Recently, there are expectations for development of an optical distance measuring sensor of an optical TOF (Time-Of-Flight) type, in order to achieve more accurate distance measurement.
There are demands for an image sensor capable of simultaneously acquiring distance information and image information by a single chip, for example, in on-vehicle use, in use in automatic manufacture systems in factories, and so on. As the image sensor is installed in the front portion of a vehicle, it is expected to be used in detection and recognition of a preceding vehicle or in detection and recognition of a pedestrian or the like. There are also expectations for an image sensor capable of acquiring a range image consisting of a single piece of distance information or multiple pieces of distance information, separately from the image information. It is preferable to apply the TOF method to such a distance measuring sensor.
In the TOF method, pulsed light is emitted from a light source for projection of light toward an object and the pulsed light reflected by the object is detected by the photodetecting element to measure a time difference between the emission timing and the detection timing of the pulsed light. Since this time difference (Δt) is a time necessary for the pulsed light to travel a distance (2×d) which is twice the distance d to the object, at the speed of light (=c), the relation of d=(c×Δt)/2 holds. The time difference (Δt) can be translated into a phase difference between pulses emitted from the light source and detected pulses. The distance d to the object can be determined by detecting the phase difference.
An image sensor of a charge distribution type has been attracting attention as a photodetecting element for distance measurement by the TOF method. Specifically, the image sensor of the charge distribution type is configured, for example, to distribute pulsed charges generated in the image sensor according to incidence of detected pulses, into one potential well during ON durations of emitted pulses and into the other potential well during OFF durations thereof. In this case, a ratio of charge quantities distributed right and left is proportional to the phase difference between detected pulses and emitted pulses, i.e., the time necessary for the pulsed light to travel the distance twice as long as the distance to the object at the speed of light. There are various conceivable methods of the charge distribution type.
Patent Literature 1 discloses the range sensor (range image sensor) of the TOF type which comprises a light receiving region, a pair of signal charge collecting regions arranged as spatially separated from each other and configured to collect the signal charge generated according to incident light, and transfer electrodes provided for the respective signal charge collecting regions and given respective charge transfer signals of different phases.